The warning is not the cause of the error, but its occurrence could help with identifying the cause of the bug. It implies that Firefox treats the touched langpack xpi as a new install.
I retried again, and can also reproduce in 63. Previously I thought that the bug does not occur in 63, because
about:config was initially not changing language. If I reload
about:config in Firefox 63, then the language becomes bad again. This difference in perceived result is caused by the transition to Fluent for
about:config in bug 1486936.
To truly verify whether the bug occurs, have step 7 after step 6: Open another new window and check whether the UI (e.g. location bar) has the expected language.
With the updated reproduction step, I managed to reproduce
I couldn't find pre-built language packs for Nightly versions of these on the CDN, so I was not able to bisect further with
mozregression. Given the bug and good/bad versions, this regression is likely caused by bug 1492459.